1 + 2: Comic Con (Comic-Con, ComicCon) is short for Comic Convention, which started out as a convention for comic book fans, where you could buy comics, meet artists and writers, dress up as your favourite characters, etc. Nowadays, it has expanded in scope, so you will see people who work in film/TV, video and board games, and so on. You may also see the term FanExpo, which is a very similar type of event.
3: In this case, attending Comic Con just means that you went to the event. I went to one in my city earlier this summer, but didn't cosplay, though I wish I had!
4: You could call it a press conference, which implies that the press (newspapers, magazines, bloggers, etc.) is there. Such information could also be revealed at a panel, a product unveiling or even a demo, especially for video games and other electronic devices.